Liquid crystal (LC) displays are widely used for laptop computers, handheld calculators, digital watches, and similar devices in which information must be displayed to a viewer. In many applications, the displays incorporate a backlight to provide the light necessary to view the display when ambient light entering the display and reflected back out of the display is insufficient.
Backlight systems typically incorporate a light source and a light guide to direct light from the source and uniformly spread it over the display. Traditionally, light guides have been provided of light transparent material which propagate light along their length through total internal reflection. The light is typically reflected off of the back surface of the light guide and towards the front surface at angles which allow it to exit the front surface of the light guide. Various reflection mechanisms are used to distribute the light exiting the guide uniformly including reflective dots, channels, facets etc.
Backlight systems which use non-collimated light sources such as fluorescent lamps, etc. also typically incorporate at least two reflectors. A lamp cavity mirror is typically used to reflect light exiting the light source in a direction away from the light guide back towards the guide. This reflector can be specular or diffuse, although it is typically specular.
A second reflector is provided proximate the back surface of the light guide to reflect light escaping from the back surface of the light guide and redirect it towards the front surface of the light guide where it can be transmitted to the viewer. These reflectors are typically constructed of a reflective white coating that also diffuse the reflected light over a Lambertian distribution.
A primary disadvantage with the conventional reflectors used in the lamp cavity and at the back surface of the light guide is, however, their relatively high absorptivities and high transmission of incident light. Typical reflectors will absorb or transmit about 4 to about 15% of the light incident upon them. The absorbed light is, of course, not available to the viewer, thereby degrading performance of the backlight.
The absorptive losses are, of course, increased with every reflection of light from the surface of conventional reflectors. With even the best conventional reflectors which absorb 4% of incident light, the intensity level of reflected light is about 81.5% after only five reflections.
These absorptive losses are also substantially increased when the backlight is used in combination with various light recycling films such as a structured partially reflective film. One micro-replicated structured partially reflective film is available as OPTICAL LIGHTING FILM from Minnesota Mining and Manufacturing Company, St. Paul, Minn.
Structured partially reflective films typically have excellent reflectivity over certain ranges of angles but high transmission over others. Micro-replicated structured partially reflective films are available as Brightness Enhancement Film, available from Minnesota Mining and Manufacturing Company. In general, structured partially reflective films redirect and transmit light into a relatively narrow range of angles while reflecting the remainder of the light. As a result, structured films transmit light and enhance brightness in backlight systems by recycling light which would otherwise exit a backlight outside a normal viewing angle.
Although recycling light in that manner is generally desired, it is a disadvantage when combined with conventional reflectors because a portion of the light which is reflected back into the light guide is absorbed or transmitted by the conventional back reflectors. Those increased absorption losses reduce the luminance or brightness attainable by this combination of the backlight system.